Well, I know that's why what few "major" characters Stephanie had, like Elizabeth and Lulu, suddenly stopped appearing after Season 33.
But, I don't know if we can put it all on that . . . I mean, a majority of the characters that we ended up knowing them as were the creations of their respective performers, yet they're in newer and capable hands: Caroll turned Big Bird from the brainless bumpkin Jim envisioned into the lovable and relatable everychild that was needed on SS, yet Matt has done a decent job in replicating that; Kevin turned Elmo into the high-voiced, giggling, love-machine that he is today, yet Ryan has done an excellent job at replicating that.
Yes, it's hard to achieve it well, let alone achieve it at all, but it's not impossible . . . we just have to keep reminding ourselves that while the performers may not stick around forever, the characters will continue to live on in spite of it.